Complementary metal oxide semiconductor (CMOS) devices such as MOSFET transistors are commonly used in high speed, highly integrated circuits. Integrated circuit manufacturers are constantly increasing the operating speed and decreasing the size of MOSFET transistors. Such improvements yield smaller, faster ICs with more functions at lower cost.
Various problems exist with scaling MOSFET devices below 0.1 microns, however. For example, with channel lengths less than 0.1 microns the required channel doping levels become very high. It is difficult to produce high doping levels with high uniformity over the surface of a wafer. Therefore, different MOSFETs manufactured on the same wafer will have very different characteristics if high doping levels are used. Also, capacitive coupling between drain and source regions of individual MOSFETs becomes significant. Problems also exist in mass producing such devices.
For these reasons, researchers have been investigating transistor devices based on the quantum behavior of electrons in very small devices. A number of such devices that exploit electron tunneling are known in the art.
For example, U.S. Pat. No. 5,705,827 to Baba et al. discloses a tunneling transistor device having an insulated gate. The transistor operation is provided by band bending in a current channel adjacent to the gate electrode, as in a MOSFET device. The drain electrode forms an Esaki tunnel junction with the current channel.
U.S. Pat. No. 4,675,711 to Harder et al. discloses a tunneling transistor using an insulated gate electrode disposed adjacent to a tunneling layer. The tunneling layer has a band gap energy different from that of semiconductor source and drain contacts. A voltage applied to the gate changes an energy barrier height of the tunneling layer, thereby controlling a tunnel current through the tunnel layer. The device must be operated at low temperature so that thermally excited carriers do not provide conduction through the tunnel layer.
U.S. Pat. No. 5,834,793 to Shibata discloses a tunneling MOSFET transistor device having an insulated gate contact. Adjacent to the gate contact is a short current channel. Source and drain contacts are separated from the current channel by dielectric tunnel barriers about 30 Angstroms thick. The device exhibits negative resistance characteristics due to discrete energy states in the current channel.
U.S. Pat. No. 5,291,274 to Tamura discloses a tunneling transistor. The transistor of Tamura has a middle layer high dielectric constant material disposed between two tunnel junctions. The middle layer is in direct contact with a gate electrode. Source and drain electrodes are provided in contact with the tunnel junctions. When a voltage is applied to the gate electrode, the electrical potential of the middle layer is changed, thereby allowing electrons to tunnel between source and drain. A problem with the device of Tamura is that current will flow to and from the gate electrode when the device is on. Therefore, the device of Tamura requires continuous gate current for continuous operation. This is highly undesirable in many applications.
In addition to the above, others have investigated the uses of single electron transistors having tunneling junctions. A single electron transistor has a very small metallic or semiconductor island disposed between two tunnel junctions having a high resistance. Source and drain contacts are made to the tunnel junctions. A gate electrode capacitively coupled to the island provides switching control. The island is made sufficiently small such that an energy required to charge the island with a single electron is greater than the thermal energy available to electrons in the source and drain contacts. The energy required to charge the island with a single electron is given by EC=e2/2C, where e is the charge of an electron, and C is the capacitance of the island. This energy requirement for charging the island is termed the Coulomb blockade.
In operation, a voltage applied to the gate electrode capacitively raises or lowers the potential of the island. When the island potential is lowered by a certain amount, electrons can tunnel through one tunnel junction onto the island, and tunnel through the other tunnel junction off of the island. In this way, current is allowed to flow through the island for certain values of gate voltage. The resistance of a single electron transistor exhibits oscillations as gate voltage changes monotonically.
Available thermal energy increases with temperature, of course, so a single electron transistor has a maximum temperature at which it can be operated. The maximum operating temperature is determined by the capacitance of the island, which is a function of the island size. For devices to operate at room temperature, the capacitance C must be less than about 10 Attofarads. Realizing such low capacitance requires that the island be very small (e.g., less than 10 nm on a side) and located relatively far from the source, drain and gate. It is very difficult to make a single electron transistor which operates at room temperature.
An important concern in the design of a single electron transistor is the resistance of the tunnel junctions. It is best for a single electron transistor to have tunnel junctions with relatively high resistances (i.e., much greater than a quantum resistance Rq=h/2e2≈26 KOhms, where h is Planck's constant). If the resistance of the tunnel junctions is too low, then the number of electrons on the island is not well defined. Operation of a single electron transistor requires that the tunnel junctions have sufficiently high resistances such that electron locations are well defined as being either in the island or outside the island. However, high tunnel junction resistance results in a high resistance between source and drain contacts, even in a fully ‘ON’ state. A high resistance limits the switching speed and increases the power consumption of the device. Therefore, single electron transistors are limited in their electrical characteristics and potential applications.
A distinguishing characteristic of single electron transistor devices is that the island can be made of semiconductor material or metal. The island does not need to be made of material having an electron energy band gap.